A heat engine is a device that converts thermal energy, an input, to mechanical work, an output. A heat engine typically operates on a specific thermodynamic cycle principle. A Stirling engine is a well known heat engine that operates by cyclic compression and expansion of a working fluid, such as air or another gas, at different temperature levels such that there is a net conversion of heat energy to mechanical work. A knowledge of the Stirling engine is helpful for an understanding the novel engine of the present disclosure.
A good source for understanding the Stirling engine is available at http://www.animatedengines.com/vstirling.shtml where an animated description of the engine is provided. U.S. Pat. No. 7,171,811, “Multiple-Cylinder, Free-Piston, Alpha Configured Stirling Engines and Heat Pumps with Stepped Pistons”, file on Sep. 29, 2005 describes related heat engines and is incorporated herein by reference.
The Stirling engine is like a steam engine in that all of the engine heat flows in and out through the engine wall. An engine having this type of heat flow is traditionally known as an external combustion engine in contrast to an internal combustion engine where the heat input is provided by combustion of a fuel within a cylinder of engine. However, unlike the steam engine's use of water in both its liquid and gaseous phases as the working fluid, the Stirling engine encloses a fixed quantity of permanently gaseous fluid such as air or helium. As in all heat engines, the general cycle consists of compressing cool gas, heating the gas, expanding the hot gas, and finally cooling the gas before repeating the cycle.
Originally conceived in 1816 as an industrial prime mover to compete with the steam engine, its practical use was largely confined to low-power domestic applications for over a century. The Stirling engine is noted for its high efficiency, quiet operation, and the ease with which it can use almost any heat source. This heat source compatibility is valuable when considering alternative and renewable energy sources. Further, such compatibility has become increasingly significant as the price of conventional fuels rises, and in light of concerns such as limited oil reserves and climate change. The engine is currently exciting interest as the core component of micro combined heat and power (CHP) units, in which it is more efficient and safer than a comparable steam engine.
It is generally not accepted that large thermal heat engines are a viable component for providing mechanical energy from low grade heat sources such as waste heat, natural sources, and solar energy from non-concentrating collectors. Heat engines are commonly considered as uneconomical as the temperature difference between a heat source and a cooling source becomes small, i.e., the difference approaches zero.
Because of this generally accepted view, large heat engines using air as the working fluid with small temperature differences are not commercially available. Even though table top Stirling engines have operated with air as the working fluid at temperature differentials as low as around 0.5 degrees centigrade with around one watt of output, essentially no effort has been put forth to build large heat engines. Small demonstration Stirling engine models are often advertised as available for purchase, but no scale up possibilities have been described in their corresponding literature or elsewhere. Prior art literature continues to instruct that large engines could be built, however the literature indicates that they would be very large and probably have little economic value. No references have been found in public domain that encourages breaking away from conventional thinking, that it is not possible to create a large heat engine that operates on very low temperature differentials that is economical.
Currently, commercially available heat engines that show promise in the market place generally operate at high speeds, high pressures, and large temperature differentials. Such engines also use a working fluid other than air. Further, the conventional approach to building hot air engines uses expensive components and features of conventional internal combustion engines, such as, for example, machined cylinders and pistons, high pressure seals, and crank shafts, etc. Thus, such a hot air engine typically has features and an associated cost of high specific energy engines, but can only perform as a low specific energy engine. Therefore, a hot air engine using components of other engines have not been viable when low temperature differential are considered. Hence, the limited numbers of successful Stirling heat engines, even operating with high grade energy sources, are typically in the range of few horsepower. Such Stirling engines are usually small and tailored to specialized applications. One exception is a large specialized Stirling engine that is used in submarines, primarily because of the quietness of the engines operation.
It is known by some in area of heat engines that the largest heat engine, in physical size, to be built was demonstrated in the 250 foot Caloric ship. Even though the Caloric ship's engine was not a low temperature differential engine it is worthy of mention herein because of its size. The Caloric ship's engine had four 14 foot diameter expansion cylinders and pistons with six foot strokes for a total expansion displacement of 3,694 cubic feet. Even though the engine propelled the ship for many hours, the engine power output was well below expectations. Even though the engine showed promise it was declared unsuccessful by the educated engineers, scientist, and those skilled in the art of that time. The novel heat engine of the present disclosure operates on low temperature differentials and may have displacements greater than that of the Caloric ship's engine.
The present disclosure identifies the Caloric ship's engine low power output problem. The finding recognized that selection of engine speed in a regenerative engine, such as a heat engine, is paramount to performance. The most important reason that the Caloric ship's engine underperformed was an unmatched load reduced the engine's speed to about one third of its design speed, thereby substantially lowering its power output. With commercialization of internal combustion engines for vehicles, the need for a transmission that permits high engine RPM at low linear speeds is well recognized. The Caloric ship's engine had no such speed converter, the transmission. Hence its performance is somewhat analogous to starting a straight shift truck in high gear. The Caloric ship's engine would certainly have performed significantly better if the load had been properly matched to the torque-speed characteristics of it's heat engine.
As the demand for renewable energy continues there is a need to consider technology that is competitive with wind turbines, photovoltaic systems, solar towers, and other known renewable technologies. One way to make the comparison is to consider the payoff from an acre of farm land as will be discussed in the detailed description. Embodiments of a novel heat engine as described herein are competitive many electrical generating technologies, particularly in the southeastern United States.